Immunization strategies based on the envelope of the Human Immunodeficiency Virus (HIV) of laboratory-adapted (LA isolates have failed so far to elicit potent neutralizing antibodies against heterologous primary (PR) HIV isolates. It is generally believed that to generate relevant antibodies to PR isolates, the envelope used as immunogen should derive from a PR isolate and its structure should resemble, as much as possible, that found on virions or infected cells. However, neutralizing epitopes on virions from PR isolates, or on cells infected by PR isolates, are not readily accessible to antibody binding because they are shielded by various envelope regions. Therefore, if the structure of the immunogen resembles that present on virions, neutralization epitopes on the immunogen will not be exposed and may not be efficiently presented to the immune system. We therefore, propose to use an envelope-based immunogen that was modified to readily present neutralization epitopes that are conserved among PR isolates. The construction of this immunogen is based on our recent observations made with a partial 30 amino acid deletion within the second hypervariable gp120 region (V2 loop) of the neutralization resistant PR virus, SF162. The introduced deletion does not effect the ability of the virus, now termed SF162deltaV2, to replicate in PBMC or modify its co-receptor usage. Of more interest; however, is the finding that now SF162deltaV2 is now highly susceptible to neutralization by various sera obtained from HIV-infected patients. Our studies indicate that the region, which becomes exposed upon V2 loop-deletion, contains epitopes located in the CD4-binding site and epitopes, which participate in HIV-cell fusion, and its structure is conserved among PR isolates. Thus, during infection this region is infrequently exposed to the immune system and the titer of antibodies generated against it is low. We believe that V2 loop-deletion permanently exposes this region. Using DNA-priming and protein boosting methodologies we propose to express in rabbits, the oligomeric form of the SF 162deltaV2 envelope and characterize the antibodies generated against it. Our hope is that this immunogen will be more potent in generating broad neutralization responses that the corresponding from the wtSF162 envelope. Therefore, the proposed study is targeted to the area of the structure and immunogenicity of HIV envelope proteins.